Locked-in oscillator circuit



July 4, 1950 B. s. vlLKoMERsoN LocKED-IN OSCILLATQR CIRCUIT 2 Sheets-Sheet 1 Filed Jan. 4, 1945 LULU Amal/)ffm July 4, 1950 B. s. vlLKoMERsoN LOCKED-IN OSCILLATOR CIRCUIT Filed Jan. 4, 1945 /NpuT 2 Sheets-Sheet 2 'fao IN V EN TOR.

14770,?/VEK Patented .uly 4, QS

LOCKED-IN OSCILLATOR CIRCUIT Benjamin S. Vilkomerson, Camden, N. J., assign or to Radio Corporation of Amerca,a corporad tion of Delaware Application January 4, 1945, Serial No. 571,222

(Cl. Z50-20) 13 Claims. l

My present invention relates generally to locked-in oscillator circuits, and more particularly to improved locked-in oscillators of the frequency divider ty-pe,

In his U. S. Patent No. 2,356,201, granted August 22, 1944, George L. Beers has disclosed and claimed a frequency modulation signal receiving system in which the frequency variation or swing corresponding to a predetermined percentage modulation of the receivedy signal is reduced prior to demodulation and reproduction of the signals. In the embodiment of the invention which Beers described, demodulation was effected by a discriminator-rectier network lwhich converted frequency variations into amplitude variations prior to detection. The Beers system specifically provided a frequency modulation signal receiving system in which the frequency variation of the received signal lwas reduced by a predetermined ratio, such as to l, before demodulation, and further provided an improved frequency modulation signal receiving system inherently responsive to frequency variations over a limited range only.

A receiving system constructed in accordance `with the Beers patent may include means for heterodyning a received frequency modulated carrier wave, or signal, to produce a corresponding intermediate frequency wave modulated over a range of frequencies. The latter is applied by Beers to an oscillator which is locked in with the frequency modulated wave over a range of frequency variations including, and preferably substantially limited to, the range of frequencies of the modulated wave. The output of the oscillator is fed to a, discriminator network. The locked-inoscillator is important in carrying out the objects of improved noise reduction and irnproved adjacent channel selectivity. The only signal which reaches the discriminator is that represented by the frequency variations of the oscillations produced by the locked-in oscillator. By designing the oscillator so as to lock in only over a frequency swing substantially limited to the range of frequencies of the desired modulated ywave, the oscillator was prevented from shifting its frequency sufliciently to lock in with interfering frequencies outsdeof such range or with an undesired signal on an adjacent channel. The locked-in oscillator produced voltages at sub-harmonics or sub-multiples of the frequencies of the intermediate or other supplied modulated wave, andthe oscillator wascoupled tiple .frequencies produced by the oscillator. Thel receiver of the Beers rpatent employed on off-y resonance discriminator circuit of the kindl shownin the U. S. Patent to John D. Reid, No.

2,341,240, granted February 8, 1944. Beers fur.

ther showed how a reactanec tube may be used to extend the lock-in range.

ance with present standards of frequency modu-v lation broadcast transmission, may have an intended frequency swing up to 150 kilocycles (kc.)

per second, I believe itto be desirable to provide a minimum lock-in range at the locked-in oscil-l, latorfof the order of i 125 kc. This provides tol-v eranceffor mis-tuning by the user, frequency drift .-.in the receiver circuits, over-modulation of the FM transmitter, etc., and thereby enables the locked-in oscillator circuits better to follow the'received wave under such conditions.

AOne of the main objects `of my present inven-v tion is to provide a locked-in oscillator system..

particularly for frequency modulation receivers,

which .provides increased sensitivity and is capA able of securingan extended lock-in range with to selective circuits in the discriminatoi` network relatively weaker signal voltages and without requiring the use of a reactance control tube or is tuned to the mean frequency of the applied frequency modulation signals.

Specifically, I utilize cascaded locked-in osev cillators, provided either by separate tubes or by a single tube, which function to provide the desired subharmonic frequency energy in a series of different frequency divisions.

fifth subharmonic of the operating intermediate frequency (I. F.) to lock in an oscillator operat-V ing at the subharmonic frequency, in my arrangement a firsty oscillator is locked in atl a` small order subharmonic frequency, and the output of the first oscillator is used to lock in a second oscillator operating at a higher order subharmonic frequency. Specifically, Whereas Beers secures a frequency division of 5:1 by tuning the oscillator to the lfifth subharmonic of the input. I. F. energy, in my circuits the first oscillator isl tuned to the second subharmonic of the I. F. for lock-in, and the second subharmonic energy of the rst oscillator is used to lock in the second,-

For exam-V ple, instead of, as in the Beers system, using the,

A gnam oscillator at the third subharmonic frequency thereof. The resulting third sub-harmonic os-V cillator energy (whose sixth harmonic is the I. F.) is detected with use of a simple discriminator circuit of the type generally related to the detector shown by S. W. Seeley in his U. S. Patent No. 2,121,103, grantedJune 21, 1938. v

vWherever the term frequency modulation is used throughout the present description and claims, it should be understood to refer to any.`

modulation where the instantaneous frequency of the transmitted Waves is varied bythe-applicationof modulating voltage of any alternating character such as music or speech.. There are many possible functional relations between the instantaneous wave frequency and the modulating voltage which are, or can be, used; For-example, if," the instantaneous frequency is caused to lsl'iiftin direct proportion to theV instantaneous value of themodulating voltage, there results one common, form of frequency modulation, or if the instan-- taneous frequency iscaused to vary as the time integral of the modulatingr voltage there results a type of frequency variation, which is usually called phase modulation because it isV with equal Regardless of the exact nature of the functional'- relation mentionedabove, however, the system of4 the present invention can be employed, and hence such terms as frequency modulation, frequency modulated, and the like should be taken inthe broad sense here defined. The generic term angle modulation comprehends frequency or phase modulations, or the `aforesaid hybrid combinationsthereof.

Still otherfeatures and objects will best be understood by reference to the followingrillustrative description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be` carried into effect;

In the drawings:

Fig. 1 is a circuit diagram `which illustrates one form of a frequency modulation signal receiving system embodying the invention;

Fig'. 2 is a circuit diagram illustrating a modified form of locked-in oscillator; land Fig. 3 shows a further modification of the circuit of Fig. 2.

Referring to the accompanying drawings, whereinlike reference characters in the different. figures designate similar circuit elements, the signal input circuits of my receiving system may be off any suitable type. Asis well known to those skilled in the art of FMreception the'receiver may employ a tunable radio frequency amplifier, a

first detector, a local-oscillatorl and an intermediate frequency (I. F.) amplifier ofthe usual superheterodyne type of vacuum tube receivercircuit.-Y A conventional dipoleantennamaybe used; asasuitable collector'of signals for the tunable; l radio frequency amplifier: As shown, theinteri mediate frequency amplifier is provided with anv 4, output transformer II) having primary and secondary circuits each resonant to the operating intermediate frequency. The primary and secondary circuits of transformer ID are suitably coupled, and any suitable means may be associated with the secondary circuit II to give the transformer IEI a` resonancel curve of the desired breadth. In the present example, the I. F. value may be assumed to be 4.3 mc. (megacycles), and the transformer I@ is designed to pass the frequency band occupied by the modulated carrier wave with %y modulation, plus the necessary toleranoefor mis-tu ning, frequency drift of receiver circuitsetc. For the frequency modulation broadcastingstandards in use at the present time, the input circuits of the receiving system, including the intermediate frequency transformer I0 with its secondary circuit II, may be made responsive over a range of frequencies of 200 to 250 kc. for reception of a carrier wave with a total intended frequency swing of the order of kc'.

It will be understood' by those, skilledl in'theart': of radio communication that the circuits prior to transformer IIB are providedwithA the usual variable tuning means. The inputn circuits of the radio frequency amplifier and first detector arel each, in the case assumed above, tunedto the mean or center frequency of the desiredstation channel. The local oscillatorA is, concurrently tuned to an oscillation frequency differing from said mean frequency by thevalue of the desired intermediate frequency. The present FM broadcast bandis 40'to 50Vmegacycles, (mc), witheach transmitter station allotteda channel Width of V200 kc.

The I. F. signal energy developed attrans'- former secondary circuit II*y is; amplified by--theA final I. F. amplifier tube I which is depicted, asA al pentode type tube. The present'inventionis not limited to thatpa-rticular type of tube; The` low potential side-v of input' circuit IIl isVV preferably-returned to groundbytheAVC; (automatici volume control) lead 2i Theplate circuit of'tubeN I includes the resonant circuit-'tunedato the operating I. F. value; which Vmayconventionallybe 4.3 mc. Then plate voltagesupply lead isA connected totheB-I- terminal, say-at) +250 volts,` of a suitable direct-currentsource: The-signal re'- turn path to thegrounded*,cathodel ofV tube Vis'l through condenseri.; The-tuned circuit' 3 isrinf ductively coupled; as indicated at M, to'V the resonant circuits'f and I7. The' circuit fis'tune'd toa frequencybfy 2.15 mc., this vvalue being; thei second subharmonic, i. e. one-half,- Vof theel;- F.L value. Resistorhin shunt with circuitwacts to' broadenitsy tuning" thereby making iteasier forthecircuitto oscillate at frequenciesother than that determined by the LC product; Thisv helpsv increase lock-inl rangeV and sensitivity, lin that I it' reduces the; minimum value of signal strength required fora givenY lock-in range; The high"A alternating potential side of circuitis connectedJ by'lead I to the plate 8 of thev first-locked-in" oscillatortube -Sr The tubel 9 isschematically represented as being a'pentagrid converter type of tube; although*V there may be employedany tubecapable of generating oscillations -atthe requiredfsubharmon-icffrequency of 2.15 mc. The plate Blisestafblishedi at asuitable positive voltage by connectin-g thei low potential side of circuit 5to the B+ terminal through resistor I. Resistor lilfiandcondenservf II form a filter to keep-thef2.15 me. oscillatorJ voltage out of the common'Bf-I- supply; Con-'i denser I vI D establishes the low-potential 'sidelofrv circuit 5 at ground potential for 2.15 mc. oscillations. The cathode I2 of tube-9 is grounded, whilefthe rst grid I 3 is connected through direct current blocking condenser I4 to the plate side of tuned circuit 3. Grid return resistor I5 connects signal input grid I3 to the grounded cathode.l The resistor I5, furthermore, functions to develop thereacross a direct current voltage by virtue of grid current .which is caused to flow in response to the impression of signals on the grid.

The control grids of prior signal transmission tubes, as grid I' of tube I, are connected by lead 2 to thegrid end of resistor I5. Suitable lter resistors I6, I6 are inserted in the AVC line to prevent the application upon the controlled grids of alternating voltages. The AVC circuit functions in the usual and well understood manner to prevent overloading of the grid I3. Strong local signals, unless compensated for by AVC, may act to develop signal voltages at grid I3 large enough to cause the oscillator to stop oscillating and to act as an I. F. amplifier.

The local oscillations of 2.15 mc. mean frequency are produced constantly by virtue of the inductive coupling M1 between the oscillator grid coil Il and oscillator anode coil 5. For best results coils I'I and 5 are closely coupled. The oscillator grid I8 is the third grid from the cathode, and is connected to the vungrounded end of coil I1. Resistor I'I, in circuit with electrode I8, acts to prevent the oscillator from oscillating at a frequency determined by the grid circuit L (inductfnce) and'C v(capacity) constants instead of the plate circuit L and C constants. The grid I8 is surrounded by a positive electrostatic field established by the second and fourth grids of the tube. The latter grids are connected through voltage-reducing resistor I9 to the B+ terminal to provide a suitable positive operating voltage for them. It will be seen that the electron stream flowing from cathode I2 to plate 8 is subjected to control by both of grids I8 and I3.

Magnetic coupling, indicated by M on the drawing,"is provided between circuit 3 and circuit 5. This coupling is sufliciently close so that by virtue of the tight coupling between coils I1 and 5', circuit 3 may be considered as coupled also to coil II. It has been found that suitable magnitude and sense of the coupling between tuned circuits 3 and 5' will cause the oscillator to lock in with about half asmuch signal input as that required 'when no magnetic coupling is used. If the coupling is in thel opposite sense, the signal input required for a given lock-in range is increased to several times that required with no such magnetic coupling. Without desiring to be limited to any particular theoretical explanation of the electrical actions involved, it is pointed out that the second harmonic component of the oscillations generated in tuned circuit 5 is of the same frequency as the signals in tuned circuit 3.

Suitable phase relationships tend to be maintained by the lock-in action, and it may be considered that in the condition of greatest lock-in range the second harmonic content of the oscillations in circuit 5, picked up by circuit 3, is fed back regeneratively through condenser I4 to grid I3'in phase with the incoming signal voltage thereby improving the lock-in action. Also, the 4.3 mc. signal voltage induced into coil I1 from circuit 3 is then impressed on grid I8 in phase with the voltage impressed on grid I3 by condenser I4. This gives yadditional change in plate current with signal voltage,- and, hence, better lock-inaction, the equivalent of using a tube of higher gm (transconductance).

The locked-in oscillator circuit associated with tube 9 functions in much the same manner as described in the aforesaid Beers patent, except that the frequency step-down ratio in this case is different, and there is employed an additional coupling between the locked-in oscillator and the signal source as explained above. Although other suitable language may be used to explain the functions of the locked-in oscillator, the following brief explanation is believed to be appropriate. Oscillator plate current pulses are modified in both amplitude and timing by slugs of space current controlled by the signal frequency at grid I3. These added plate current slugs, if they are nearly (but not exactly) in phase with the plate current pulses produced by the oscillating circuit, either accelerate or retard the oscillator frequency thereby locking it in with the signal frequency, so that at any instant the oscillator frequency is one-half of the signal frequency and the change in oscillator frequency is one-half of the change in signal frequency.

Hence, at modulation the total frequency change of the 2.15 mc. oscillations in circuit 5V is 75 kc., as compared with the 150 kc. change in circuit 3. For example, if at any instant the received signal has been heterodyned to an intermediate frequency of 4.375 mc. (up 75 kc. from the mean or center frequency of 4.3 mc.) the frequency of the locked-oscillator will then be 2.1875 mc. (up 37.5 kc. from the mean sub-multiple frequency of 2.15 mc.) thereby preserving the 2:1 ratio in frequency. Thus, there occurs inthe circuit 5 a frequency deviation from the mean frequency of i375 kc. for full modulation, assuming that the applied signal has a maximum frequency deviation of +-'75 kc.

The diminished frequency step-down ratio used in each frequency division stage of the cascaded system of my invention contributes to the rela tively greater lock-in range secured. In the af oresaid Beers system, wherein a 5:1 division ratio is used, part of the positive half of every fth cycle of signal voltage affects and controls the lockedf in oscillator. In my system every second signal voltage peak affects the first locked-111 oscillator 9 thereby giving greatly increased control of the oscillator frequency by the signal frequency. Using 2:1 division, lock-in ranges of 350 to 400 kc. have been secured using a discriminator of the type shown in the aforesaid Seeley patent with a 6SA7 type tube. i

The output of oscillator 9 is fed to a second oscillator tube 20, shown by way of example asy being of the pentode type, whose signal input grid 2i is connected by direct current blocking condenser 22 to plate 8. Hence, FM oscillatorv energy of mean frequency 2.15 rnc., is applied to grid 2l. Tube 2U may be of any other suitable type. The cathode 23 is grounded, while plate 24 may be connected to a +250 volts point on the power supply. The plate circuit includes the parallel resonant circuit 25 tuned to 716.67lsc., i. e. the third subharmonic frequency of 2.15 mc. Circuit 25 is the primary circuit of the discriminator sec tion of the FM detector.

The second grid 26 of tube 23 functions as the anode of the oscillator section of the tube, and is connected to the high alternating potential side of resonant tank circuit 2T which, in the case assumed, is also tuned to 716.67 kc. The lowv potential side of tank` circuit 21 is return-ed toground for alternating currentsy by means 'of appearing at output circuit 25 is i125 kc.

the f. ley-pass;v condenserl 28,. while it isereturned'i to; the B-lterminali through.voltageereducirrg.. re:-

i sistor'Z.. Resistor 39 shuntsitank circuit 2li to broaderrthezresponse curve thereof,` for' reasons more'f fully.' explained. in` connection with; resistor.

C 'constants instead! of the plate.` circuit 'Landi C constants.- Resistor 33fis-a gridv leakresi'stor for'- oscillator bias.v

. vCoil-i325 is magnetically and closely.. coupledlto the tank'. circuit 2l as indicatedr atlM2. Resistor 33 `1is shunted by-condenser 341s() as.v tof establish ther-lower.y end of' coil-32 at ground potentiai for alternating currents.V The coupling- Mt is so chosen that the oscillator electrodes; 2i and" 26ofthel tube, under the control .or tank-.circuit 21", provide-local oscillations at the. predetermined mean frequency of '716,67 kc; The application of the rstlocked-in' oscillator energy.; whose mean frequency is 2.15"'mc., to gridficauses the third subi-harmonic oscillationsthereoi tof' be locked with Athe applied FM energy;

Though -it=is-not essential,v for convenience :circui-tslSlandEgasrwell as all other resonant circuits inl-the disclosed embodiment cff'rny inven'- tion, employ variable magneticl'core l inductances.;

Forreasons more` full-y explained'- in connection with theefirst locked-in oscillator 9; the meanv ire'- quency of"v the oscillations produced byV tube-2li is not-Yonlyone-third oithe frequency applied to grid'i but the-frequency sWin-g'ofsuch oscil'- lationsis-falsoone-third thatof the applied` en- In other words, the frequency' deviation from the mean frequency ofthe FM "Wave energy It will,y therefore, be. seen that between the input circuit i l andthe output -circuit 25 there has loon curred a,- frequency division down to' one-sixth of the initial intermediate frequency, with concurrent reduction of thetotal frequency change by the same factor of 6.

The reasons for using cascaded 2:1 andl Szi divisions are-various. For one thing; a system loew-insensitivity" of.i the'. second.Y locked-in `osclllatoncan'bereadily controlled to make thezsylse tenr responsive.v to the. bandi width. covered by thefrstloscillator. It'isdeSirable that the strong` oscillator which." drives-:the: discriminator" operate onza: frequency Well. removed from that of the Ii. amplifier: thereby minimizing thev stability.` problem;v Inrtheembodiment of my invention described;herein; I.. E; feedback from' the Asecond locked-.in .oscillator will :be had lonlyjfrom a1 relatively weah higher-order harirnonic,. the'. 6th, rather-"i4 thanf strongr low-order harmonics. i The low-'ordervf (2nd). harmonic I. F. feedback from thefrst'lockedinoscillator is Weak becausethe fundamental.'oscillator.v output is relativelyI weak.

One ofitlie important advantages-of myv pres-y enti invention. resides in' the n.fact that there 'may readilyfbe.utilizedlan. FM detector circuit ofthe aforementionedSeeley-type. As isv Well known to those: skilled' in the-art,- this type'of detector circuit employs.- a discriminator sectionwhose tuned circuits are resonate'd tovthe desired .mean irequencyaofzthe applied FMfen'ergy. In accordanceb witlrthe.A present invention, the natureA of the.' discriminator circuits hassubstantia'lly no eiiectzon the lock-in rangeofy the :locked-in oscillater system'. Henceyitis unnecessary to employ discrimina-toricircuits which are inherentlyf critically. misaligned' relative V*toy the mean frequency oiitheroutput/of. the second locked-inA oscillator tube.,V

The discriminator-rectier network shown herein isfth-at. disclosed and claimed by WVRz Koch .irrxhistapplication Serial No. 529,074, filed using- 2:`1 division and 3:1 division, which gives anA overall frequencydivision of 6:1, is a system using'small integral steps, hence affording great sensitivity. There are severalk other advantages which accruewith the present two-stage method;

locked-in oscillators Eland 2Q, and in particular of the `coupling between the circuits ofthel in- 1 dividual oscillatorsand the operating potentials applied to the tube electrodes, their output voltages can be made substantially independent of the 'amplitude of the signal Avoltages impressed upon them. More specifically, theoutput ofthe first oscillator is nearly constant despite vari'-V ations in the strength of the receivedV signals; and by impressing itsalrnost uniform outputupon the second 'oscillator a veryhigh degree of con'- stancy of amplitude ofthe voltages impressed on `'they ultimate discriminator is obtained.

Another advantage isegainedfin' that the first oscillatorv can be made'relatively weak, and the second oscillator strong. This is desirable as the strong'second oscillator feeding the discriminator gives good audio frequency output,wherea`s greater lock-in sensitivity may be obtained with a weak first oscillator'. By adjustingthecoupling.' between-the twoilocked-in oscillators; the

` By proper choice of the circuit constants of the A.pril..1,"..1942i;` now Patent Ndl 2,410,983 granted Nov. 12, 1946:" Since. v'ther vdiscrirninator-rectiier circuit is fnot'atpart-of the present invention', it is: notzbelieved': necessaryfto. describe: it in detail.` Itis.v suiiicientforV the purposes.: ofv this applica;-` tionilto point outf that fthe' .primarycircuit 25=is in'ductive'ly:coupled,` as at M3', to the secondary coil Ml. The=latteri`s tuned'byithe condensers 4l and.' 42; which 1 are r connectedr in serieseshunt a'crossxcoil'.l 40, .tolthe same frequency to which circuit-is ztunedi; e; Y'11 6.6"7Fkc:v The highy alter'-y natingg potential. side f of'. circuit f 25- isconnected to therjunction of. condensers 4I andv 42, which are1'preferably: equal inun'agnitude;i The oppositesl ends :ofi 0011.40 areconnected to respective anodes ofthe'vpar. of .opposed .diodes 43 and 44. The. cathodesof the-'opposed diodes are connected to groundfor high frequency alternating currents by.4 condenser 45. 'd l Load resistor 4fshunts the space current` path ofdiode 43;.andloadres-istor 4l shunts the space current. path of-.diode 44. The. audio -frequency voltage f is Ktaken 01T. from.l the f. cathode'L end of res-istorAG, In generaLit can bestatedfthat when the Eli/[oscillatory energydeveloped across circuit 25 has an" instantaneousv frequencyA of 'H567 ke., .there will .be developed .equal rectified voltages.acrossresistors 46"a'nd 41. Sincethes voltages .are of 'opposite'polarity with respect" to ground, ,the instantaneous outpt'at that fre; quencywill be zero. However, as the applied energy deviats or'swihgsfronitlie'rnan fre-N quencyoi`"'ll6f67"'kc:, the net `rectified vlt'a'ge'at the'cathod end of resistor Mifwill vary inniag'- ni'tude andpolarity in"acco1'dance 'with the exitent and direction "off the aforesaid" frequency deviationA or swing. It is`t0" be` clearly 'under stood that the` specific r'Kochfcircuit shown here inmayibelreplace'd' bythe circuit shown in the aforesaid Seeley patent, or by-any other 'suitable discrhninator circuiti- -1 1.

It is likewise apparent that intermediate frequencies other than the 4.3 mc. value can be used. In the receiving system of this invention, the received FM signal does not pass through the receiving system to the discrirninator section in the conventional manner. The tubes 9 and 20 produce oscillations whether or not signals are being received, and the only effect of a received signal is to shift the frequencies of the cascaded locked-in oscillators in accordance with the modulation thereby causing the frequency of the voltage applied to the discriminator to vary in accordance with the modulation of the received signal but with a greatly reduced frequency swing. If oscillations are not produced by the oscillators the system is inoperative to receive signals, inasmuch as energy at the intermediate frequency impressed on the grid I3 cannot pass through tubes 9 and 20, and even if it could the selective circuits 40 and 25 designed to operate at mean frequencies of 1/6 I. F. could not respond to the signal I. F.

By the use of the circuits described above, satisfactory lock-in of the cascade oscillator over a minimum range of i125 kc. has been obtained with a signal potential applied to control grid I' of approximately 0.0015 volt. This results in from 10 to 20 volts audio output from the FM detector. It will be noted that this voltage gain is obtained at frequencies which differ from either that of the radio frequency or intermediate frequency circuits. This contributes materially to the overall stability of the receiver as previously discussed. Sinceone of the major problems in frequency modulation receivers is to obtain high sensitivity or gain with stability, and since the gain which lcan be obtained with reasonable precautions at any specic frequency is definitely limited, the additional gain obtained at a considerably lower frequency is important.

Another advantage of the present receiving system, which has also been described in the aforesaid Beers patent, is that because the discriminator operates at a much lower frequency than in conventional receivers, the circuit const/ants can be more readily controlled since the frequency band over which the circuits operate is correspondingly reduced. Also my invention retains in important degree the improved adjacent channel selectivity obtained by the Beers arrangement. In a conventional frequency mod-- ulation receiver, the adjacent channel selectivity is determined by the response curves of the radio frequency, intermediate frequency and discrimf. inator circuits. If the signal on the channel adjacent to a desired signal is sufficiently strong to produce a, voltage of a certain value at the discriminator-rectifier network, interference with the desired signal will be obtained even though the frequency deviations of the undesired signal do not operate over the useful portion of the discriminator characteristic, In the receiver of this invention the only signal which reaches the discriminator is represented by the frequency variations in the voltages produced by the cascade locked-in oscillator. It is desirable in a receiver as shown in Fig. 1 that the oscillators 9 and be locked in with the intermediate fren quency wave over the full rangeV of 100% modulation, but by limiting the lock-in range of my.

cascade oscillator to the useful band width of the discriminator characteristic and to the eX- pected frequency variations of the received wave under various conditions of operation the selectivity of the receiver is materially increased without resorting to the use of additional selective circuits. However, as previously indicated, I prefer to employ a lock-in range, of adequate Width to guard against mistuning with mechanical and/ or remote tuners, to help overcome frequency drift due to temperature and humidity changes, and to allow for any likely degree of over-modulation at the transmitter.

While I do not desire to limit the invention to the following table of constants, the latter is given to enable those skilled in the art of FM re, ception most readily to'construct a receiver in accordance with my invention:

C14=91 micromicrofarads (mmf.) C22=3.4 mmf.

C2s=0.01 mfd.

C34=330 mmf.

C41, C4z=270 mmf. each R1oo=2.2 kilo-Ohm's Rs=15 kilo-ohms R15=390 kilo-ohms R19=82 kilo-ohms lit-29:56 kilo-ohms R3n=3 kilo-ohms Ras=18 kilo-ohms R31=2.2 kilo-ohms R46, Rit/: kilo-ohms each The functions of the cascaded local oscillator tubes of Fig. 1 may be` performed with a single tube. Figs. 2 and 3 show respectively different embodiments of such single tube systems. In Fig. 2 tube 50 may be a pentagrid type tube whose first grid 5I is connected by coupling condenser 52 to a source of 4.3 mc. FM signal. The cathode 53 of tube 50 is connected to ground through coil 54, and resistor 55 returns grid 5| to the grounded end of cathode coil 54. The second and fourth grids are tied together to function as a single electrode 56. The latter acts as the anode of an oscillator whose tank circuit 5'lis connected between a suitable point S+ of positive potential and the electrode 56. The coil 54 is regeneratively coupled, as by inductive coupling M4, to tank circuit 51. Tank circuit 5l is tuned to the second subharmonic frequency,y 2.15 mc., of the input frequency of 4.3 m'c. The application of 11.?. energy at 4.3 mc. to the grid 5i `causes lock-in of the oscillations of 2.15 mc.

The second locked-in oscillator section of tube 50 comprises cathode 53, the third gridl 58 and plate 59. Grid 58 is located between the elements of electrode 5E, and, therefore, electrode 56 acts as a positive screen for oscillator grid 53. The plate 59 is connected to a suitable positive voltage point B+ through the coil of tank circuit 68. The latter is tuned to the third sub-harmonic (716.67 kc.) frequency of 2.15 mc. Grid 58 is connected to the upper endl of coupling coil 6l, while the lower end thereof is connected to ground through grid return resistor 52 shunted by condenser 63. The coil 6l and tank circuit 60 are inductively coupled, vasat M5, to provide oscillations at 716.67 kc. The locked-in oscillations developed at circuit $0 may be transmitted to the following discriminator section in the manner shown in Fig. 1.

The system of. Fig. 2 acts in the following manner: Cathode 53, grid 5l and screen electrode 56 cooperate to provide the elements of a triode locked-in oscillator operating at 2.15 mc. The 4.3 mc. signal voltage being in series with the voltage induced in -tickler coil 54 .between cath;- ode and grid 5l, the 4.3 mc. signal locks in the 2.15-rnc. oscillations. Avirtual cathode is Vformed between vvthe portion-of grid Et-nearest the cathode and grid 59. This virtual cathode, grid 58, screen 56, the suppressor grid and plate 59 -form the equivalent of la pentode, whicl'nby virtue of coupling M5, acts as the second locked-in oscillator operating at 716.67 kc. The coupling of the second oscillator to the first -is due to the modulation of the virtual cathode by the rst oscillator, i. e., by electron coupling and also by capacity between electrodes in the respective oscillator sections. Otherwise, the functioning of the system of Fig. 2 is the same as in the twotube cascaded system of Fig. 1.

The modification shown in Fig. 3 .employs tube 10, which may take the form of a GAS type of tube or any other form providingr an auxiliary anode in addition to the main anode.` The ,cathode Il, rst grid 'l2 and second electrode .(auxiliary anode rod in the 6A8) 13 cooperate to provide the irst locked-in oscillator section. The 4.3 mc. signals are applied to the resonant input circuit l5, Whose high alternating potential side is coupled to control grid 12 by condenser 1.4. The grid return resistor I4 connects grid 'l2 .back to ground. The low potential side of circuit ,115 is .connected to ground through coupling coil Till. 'Il-he latter is regeneratively coupled to resonant tank circuit 'l'l which is tuned to 2.15 mc. The tank circuit is included in circuit with the oscillator anode electrode 73, and the latter is shown connected to a lpoint of positive potential of +105 volts. The I. F. signals applied to grid 'I2 from input circuit 'l5 act to lock in the locally-produced oscillations of 2.15 mc.

vThe second locked-in oscillator section comprises cathode `7|, the fourth grid 18 and plate 19. Plate 19 is regeneratively coupled to the tank circuit 80 by means of the inductive coupling be- -tween coil 8l and tank circuit 89. The plate 19 Yis connected through coil 8l to a potential point of +225 volts, while grid 18 is connected through condenser 82 to the high alternating potential side of tank circuit `80. Tank circuit 80 is tuned to '1116.67 ko. to causo .production of oscillations of lthat frequency. The grid return resisto-r l83 shunts condenser 82 to ground; and the high potential side of plate coil 8| may be connected to 'a discriminator, as, for example, disclosed in Fig. 1. The d iscriminator connection could, also, be taken `frorn the ungrounded side of circuit 80. The control grid 18 is located between the positive shielding grids 90, the latter being effectively at +95 volts by virtue of the insertion of the voltage-reducing resistor 9| in the +225 volts line.

The '716.67 kc. oscillations produced in the second looked-in oscillator section of tube 16| are locked in over a $12.5 kc. deviation range `by the 2.15 mc. oscillations. T'he explanation given above relative to the functioning of the circuit of Fig. 2 applies equally well to Fig. 3. However,

because of the availability of a separate plate for the rst lockedin oscillator triode in Fig. 3, coil 54 of the system of Fig. 2 is taken out of the cathode circuit and placed directly in series with the I. F. signal input circuit.y Coil 16 of Fig. 3 shows this change. Not using the screen 9D as the triode plate (unlike Fig. `,2) and bringing the cathode 1I directly to ground make the tube more nearly simulate two separate tubes. In; the rrlodification of Fig. '3, only electron coupling exists between the two locked-in oscillators since electrode 90 provides a shield between them.

By way of illustration only, the following vlist i`12 of constants is Agiven for an actual embodiment of the -system of f-Fig. 3:

It has 'been foundthat when a locked-inoscillator is used in accordance with vthis invention, the amplitude of .the output of the oscillator` is practically constant Awhether or not va signal fis present'and regardless ofatheamplitude of the in.- comi-ng signal. This .means that there is :practically no amplitude-modulation (AM) in its output, and a lsingle .grounded-cathode diode lassociated with .a discriminator network will give satisfactory performance. lThis is an important commercial advantage, since this .makes it possible =.to use, as the FM detector in AM-FM receivers, the grounded-cathode ldiode tube which `is normally .employed almost universally as an AM detector. Circuits may therefore vbe simplified, and the cost of an extra FM detector -tube may 'be avoided.

While 'I have indicated and described sevveral systems for carrying my invention into effect, it iwill 'be .apparent Vto one skilled in the vart that my invention is by no means limited to the particularorganizations shown and described'but that :many modifications may be made without departing from the scope of my invention.

vWhat AI claim is:

l. `In a -frequency modulation receiving system, a circuit for receiving' frequency modulated waves, Vmeans for supplying frequency modulated signal waves to said circuit, a demodulator, selective circuits in the input of the demodulator for converting frequency variations to amplitude variations, and means connected'between said receiving circuit and said selective circuits for improving the signal to noise ratio of the system, said ylast-named means including a rst oscillator locked in step with the signal input frequencies, saidflrst oscillator including an amplifier having atleast one control grid, means coupled to said circuit Afor impressing said signal waves Aon said control grid, and means for impressing a predetermined subharmonic frequency of said signal waves on said control grid, a second oscillator locked in step with the first oscillator output signals, said oscillators being constructed and arranged to operate over widely spaced lock-in frequency ranges, and means for coupling the output of Athe second oscillator to said selective circuits. l s

2. In a frequency modulation receiving system, an input circuit for receiving frequency modulated waves, means for supplying frequency modulated signal waves'to said circuit, a demodulator, selective circuits in the input of the demodulatoi` for converting frequency variations to amplitude variations, a first vacuum tube containing an anode, cathode and a plurality of grids, means for impressing frequency modulated signal waves from said input circuit on one of said grids, an oscillator including said anode, another of said grids, and a tank circuit containing inductance and capacity tuned to a predetermined sub-harmonic frequency of the signal frequency, means coupling said oscillator tank circuit and the input circuit for causing the oscillator to lock in with a desired frequency modulated wave over a range including, and substantially limited to atraves the range of frequencies of such modulated wave under various, anticipatable operating conditions, a second tube containing an anode, cathode and at least two grids, means coupling the latter anode to said demodulator selective circuits, a second oscillator including the two grids of thc second tube and a second tank circuit tuned to a predetermined other subharmonic frequency of the first subharmonic frequency, means applying locked-in oscillations of the first oscillator to one of the two grids of the second tube, and said demodulator selective circuits being tuned to said other subharmonic frequency.

3. In combination, a vacuum tube containing an anode, cathode and a plurality of grids, means for impressing angle modulated signal waves on one of said grids, an oscillator including said anode, another of said grids and a tank circuit connected to the anode and containing inductance and capacity, said tank circuit being tuned to a predetermined sub-harmonic frequency of the frequency of said signal waves, and means 'providing substantially close magnetic coupling between the oscillator tank circuit and said impressing means for causing the oscillator to lock in with a desired signal wave over a range of angle variations.

4. In an angle modulated carrier wave reciving system, a receiving circuit, means for supplying to said circuit waves modulated over a range of frequencies, a demodulator having an input network responsive to frequencies which are submultiples of the rst-mentioned frequencies and having a pass band of a width bearing substantially the same ratio to the width of said firstmentioned range of frequencies as sub-multiple frequencies bear to the first-mentioned frequencies, cascaded locked-in oscillation-producing means interposed between the receiving circuit and said demodulator input network, said cascaded oscillation-producing means including a tube having a control grid on which said waves are impressed, and means for impressing energy at a predetermined sub-multiple of said first mentioned frequencies, said cascaded oscillationproducing means impressing voltages on the demodulator input network at frequencies which vare sub-multiples of the first-mentioned frequencies and which vary over a correspondingly (u reduced frequency range, and means including said means for impressing for causing the separate oscillation-producing means to lock in with a desired modulated wave at different predetermined sub-multiple frequencies.

5. In a frequency modulated carrier wave receiving system, a receiving circuit, means for supplying to said circuit signal waves modulated over a range of frequencies, a demodulator having an input network responsive to frequencies which are sub-multiples of the rst mentioned vfrequencies and having a pass band of a width bearing substantially the same ratio to the width of said first-mentioned range of frequencies as said sub-multiple frequencies bear to the firstmentioned frequencies, a vacuum tube interposed between the receiving circuit and said ydemodulator input network, and said tube having a pair of oscillator sections employing the common electron stream of the tube, one oscillator section including a iirst tank circuit tuned to a rst submultiple frequency of the signal waves, the second oscillator section including a separate tank circuit tuned to a different sub-multiple frequency of the first sub-multiple frequency, said demodulator input network being tuned to the frequency of said second oscillator tank circuit.

6. In a frequency modulation receiving system of the type comprising a receiving circuit for waves modulated over a range of frequencies, means for reproducing the received signals, and means for discriminating in favor of a desired signal and against interfering noise and undesired signals; the improvement in said last means which comprises cascaded separate locked-in oscillators tuned to different sub-multiple frequencies, said oscillators each being constructed and arranged to operate over widely spaced lock-in ranges one of said oscillators including an amplifier having input and output electrodes, means coupling said circuit to said input electrodes for impressing said received signals thereon, and means regeneratively coupled to said amplifier for further impressing energy at a predetermined one of said sub-harmonic frequencies on said input electrodes.

7. In a frequency modulation receiving system, a circuit for receiving waves modulated over a range of frequencies, means for reproducing the received signals, and interference preventing devices comprising a first oscillation-producing means constructed and arranged to lock in at a predetermined submultiple of the frequency of the desired modulated wave and over a predetermined range of frequencies, said first oscillation-producing means including an amplifer having at least one control grid, means coupled to said circuit for impressing said modulated wave on said control grid, and means for impressing said predetermined sub-multiple frequency on said control grid, and means for amplifying the output and reducing the susceptability to noise pulses of the oscillation-producing means comprising a second oscillation-producing means constructed and arranged to lock in at a different'sub-multiple frequency with the output of the first oscillation-producing means over a widely spaced frequency range.

8. In a locked-in oscillator system, a tube having a cathode, anode and at least three auxiliary electrodes, a first tank circuit regeneratively coupling the cathode and two of said auxiliary electrodes to provide a first oscillator, means for applying frequency-variable Waves to said first oscillator, said tank circuit being tuned to a desired sub-multiple frequency of the mean frequency of sai-d waves, a second tank circuit regeneratively coupling the cathode, anode and the third auxiliary electrode of the tube to provide a second oscillator, and means for tuning the second tank circuit to a desired different submultiple frequency of said rst sub-multiple frequency. e

f 9. In combination, a vacuum tube containing an anode, cathode and a plurality of grids, means for capacitatively impressing angle modulated signal waves on one of said grids, a locked-in oscillator including said anode, a second of said grids and a tank circuit connected to the anode and containing inductance and capacity, said tankcircuit being tuned to a predetermined subharmonic frequency of the frequency of said signal waves, means providing magnetic coupling between the oscillator grid and said impressing means, the alternating voltage on said two grids being in phase, and said two grids providing a double control of the space current thereby to improve lock-in sensitivity.

10. In a frequency modulation receiving system of the type comprising a circuit for receiving frequency `modulated Waves, -means .for :supplying frequency modulated signal Waves to .said circuit, a demodulator, selective circuits lin the input ofv the demodulator for converting irequency variations to amplitude variations, and connections between said receiving circuit and said selective circuits, said connections including means for increasing the selectivity of vthe system; the improvement which includes said lastnamed means comprising a first relatively Weak oscillation-producing means locked -in step With a desired frequency modulated wave over va range of frequency variations, and a second relatively strong oscillation-producing means locked in step With the oscillatory output of the first oscillation-producing means each of said oscillationproducing means including an amplifier having input .and output electrodes, means lfor impressingr said desired frequency modulated -Wave to said input electrodes, and means regeneratively couple to said amplifier for further impressing energy at a sub-harmonic of said Awave on said input electrodes.

`11. A system for amplifying angle-modulated Wave energy which achieves 4high sensitivity and gain with inherent stability, said system com prising a plurality of cascaded locked-in oscilla-` tors, a first one of said oscillators being operative at small amplitudes and at a frequency which is a low, integral, subharmonic of the Wave :mean frequency thereby to provide high sensitivity and gain, said first one of said oscillators comprising l a tube having a cathode, an anode and at least tWo .control grids, means for impressing said Wave energy on one of said grids, a resonant cir.- cuit tuned to said low sub-harmonic frequency and coupled to both of said grids and to said anode to impress energy atsaid low sub-harmonic frequency on both of said grids, and at least yone succeeding one of said locked-in oscillators being operative at a higher amplitude and at a frequency which is a higher order sub-harmonic of said Wave mean frequency, said last named oscillator stage being so designed -as to harmonic content 'and so far removed in 'means .frequency from the fwave input frequency that the only possible feedbackfmm said succeeding -oscillator .to said one oscillator is from weak, high-order relatively easily controlled harmonics of saids-ubharmonic mean frequency as compared With feedback of fundamental frequency output of amplitude modulated amplifiers, thereby to secure stability.

12. A system for amplifying angle-modulated wave energy which achieves high sensitivity and gain with inherent stability, said system 'com- Aprising successive locked-in oscillators, a rst one of said oscillators being operative a-t ysmall amplitudes and at a frequency which is a low, integral, sub-harmonic of the Wave mean frequency thereby to provide high sensitivity and gain, said first one of said oscillators comprising a tube having a cathode, an anode and at least two control grids, means for impressing said wave energy on one of said grids, a resonant ycircuit tuned to said low sub-harmonic frequency and 1-6 coupled `to, both of-,said grids and to said .anode to impress energy at sai-d .low sub-harmonic fre` quency. on -both of said grids, lsaid .successive locked-in oscillators being respectively .operative at yfrequencies Which are successively of higher order sub-harmonics of said Wave mean frequency, ksaid successive Voscillators each ,being .so designedas -to harmonic `content and so vfar removed :inv mean frequency from the -Wavie .input frequency thereto that the only possible feedback to the -rst oscillator Vfrom the second one of said successive oscillators `is Afrom Weak, yhiglmorder, relatively easily controlled .harmonics .of .the A4respective mean sub-harmonic frequencies as `compared with Vthe feedback of fundamental output of amplitude :modulated amplifiers, thereby to secure stability.

13. In a frequency modulation receiving .system, an input circuit forV receiving frequency modulated Waves, means for supplying frequency modulated signal waves to said circuit, a demodulator, selective circuits in the input of the demodulator for converting frequency variations to amplitude variations, a nrst vacuum tube lcontaining an anode, cathode and `.a plurality .of grids, means for impressing frequency modulated signal Waves from said input circuit Ion .one .of said grids, an oscillator including said anode, another Vof said grids, and a tank .circuit containing inductance and .capacity tuned to a :predetermined Asub-harmonic frequency of the ysignal frequency, means coupling the oscillator tank circuit and `the input vcircuit for causing the os# cillator to lock in with .a desired Yfrequency modulated Wave .over a Arange of frequencies of .such modulated Wave, the .constants of said oscillator being chosen to provide v.relatively Weak oscillations, a-second tube containing an anode, .catho-de and at ileast twogrids, means coupling the latter anode to .said ydemodulator vselective circuits, a second oscillator including the second tube tWo gridsand a second tank circuit tuned to a predetermined `second subeharmonic frequency of the iirstsub-harmonic frequency, means applying locked-in oscillations of the first oscillator to one yof the two gridsof the second tube, said second oscillator providing relatively stronger oscillations, and said demodulator selective circuits.

being tuned to said second sub-harmonic quency.

free' BENJAMIN S. VILKOMERSON. n

REFERENCES CITED The following references are of record in the file of this patenti UNTIIDv STATES PATENTS 

